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Webster 1913 Edition

Noun.

One of the rings on the top of the saddle of a harness, through which the reins pass.

terret ‎(plural terrets)

One of the rings on the top of the saddle of a harness, through which the reins pass.
- 1877, Anna Sewell, Black Beauty Chapter 23
  She had a good idea of what was coming, and the moment York took the rein off the terret in order to shorten it, she took her opportunity, and reared up so suddenly, that York had his nose roughly hit, and his hat knocked off

terret